1. Field of the Invention
The present invention relates to the measurement of the roll angle of a projectile.
To guide a projectile or a missile in flight, it should be possible to take appropriate action on its steering devices, namely its rudders, thrustors, gas jets etc., and therefore it is necessary to know their orientation with respect to a plane passing through a horizontal or vertical axis and the longitudinal axis of the projectile or missile, namely the roll angle.
2. Description of the Prior Art
The roll angle can be obtained by a measurement on board the projectile or missile, for example by an inertial measurement, but this necessitates the installation of sophisticated equipment on board the projectile and gives rise to a variety of drawbacks:
penalties in terms of weight and volume; PA1 fragility of the measuring device which does not withstand very high rates of acceleration; PA1 time needed for starting up; PA1 high cost for an expendable device; PA1 the need for preventive maintenance; PA1 the need to transmit the information to the firing station if it is on the ground.
There are known ways, described notably in the American patent US-A-4.072.281 for making a measurement, from the ground, of the roll angle of a projectile or missile optically by fitting out the rear of the projectile with a retro-reflector preceded by a polarizer, illuminating the rear of the projectile from the ground and making a measurement on the ground, using a light analyzer, of the direction of polarization of the beam reflected by the rear of the projectile.
By virtue of its principle, this optical measurement of the roll angle of a projectile has an ambiguity of .pi.. There are known ways of removing this ambiguity that either rely on prior knowledge of the initial roll angle or consist of the addition, to the rear of the projectile, of an additional optical device such as a reflecting dihedron that is unmasked with respect to the illumination source only once every roll rotation.
Prior knowledge of the initial roll angle is not always available, especially when the projectile is a projectile launched by a gun.
The addition of a supplementary optical device increases the cost of the expendable parts placed on board the projectile or the missile.